For many years it has been a common practice of bicycle manufacturers to provide, as original equipment, ratio-changing power transmissions in the power train between the pedal crank and the traction wheel of the bicycle. The power train of most currently manufactured bicycles comprises a chain and sprocket drive between the pedal crank and the traction wheel. As used herein, the gear ratio of the transmission is the ratio of the diameter of the drive sprocket on the pedal crank to the diameter of the driven sprocket on the traction wheel and produces a speed ratio of the same value between the drive and driven sprockets. In bicycle parlance, the lowest speed ratio, i.e. lowest speed range, is called "first speed" (or "first gear") and higher ratios are called "second speed", "third speed", etc. A bicycle provided with a transmission having three different speed ratios is referred to as a three speed bicycle, one having ten different ratios is referred to as a ten speed bicycle and so on. Bicycles are commonly provided with three speed, ten speed, fifteen speed and twenty speed transmissions. A three speed bicycle usually has a barrel (or drum) power transmission in which the ratio changing mechanism is contained within the hub of the rear traction wheel. For five speed bicycles and those having a higher number of speeds, the different speed ratios are provided by one or more sprocket wheels (herein called "sprockets") on the pedal crank ("pedal sprockets") and a plurality of different size sprockets on the traction wheel ("wheel sprockets"), the pedal sprockets and wheel sprockets being coupled by an endless chain. The ratio changing is accomplished by a mechanism called a derailleur which controllably shifts the chain from engagement with one sprocket to another. A manually actuated shifter lever, usually mounted on the front part of the down tube or top tube of the bicycle frame, is connected by a cable to the derailleur. In the case of a five speed bicycle, it is usual practice to provide a single pedal sprocket and five different wheel sprockets and a rear derailleur for shift control. For a ten speed bicycle, there are two pedal sprockets and five wheel sprockets with both a pedal sprocket derailleur and a wheel sprocket derailleur, thus providing ten different sprocket combinations.
In the conventional bicycle transmission shifter, the pedal sprocket derailleur comprises a laterally movable chain cage which urges the chain into alignment with a selected one of the sprockets at the pedal crank. The wheel sprocket derailleur comprises a guide sprocket and tensioning sprocket which successively engage the chain in its slack portion (as distinguished from its driving portion) between the pedal sprocket and the wheel sprocket. The wheel sprockets are axially aligned and laterally displaced from each other with progressively larger diameters from the outboard to the inboard sprocket. The derailleur is provided with a cable operated transfer lever, typically a spring-loaded bell-crank lever, which shifts the guide sprocket laterally so that it can be aligned with any one of the wheel sprockets.
Although the conventional derailleur leaves much to be desired, it is the predominant type of bicycle shifter mechanism. Shifting by the rear derailleur is accomplished by repositioning the selector lever during pedalling to move the guide sprocket, and hence the chain, in either inboard or outboard direction to the adjacent sprocket. This lateral movement of the guide sprocket leads the chain from meshing engagement with one sprocket to a position in which it picks up a tooth of the adjacent sprocket and becomes meshed therewith. The shifting is controlled by the rider mostly by the feel of the selector lever and the resultant interaction of the chain and sprocket, rather than by visual positioning of the lever.
For the skilled rider on a familiar bicycle with a well adjusted derailleur, manual shifting usually results in a smooth transition from one speed to another. For a rider of lesser skill, especially with an unfamiliar bicycle and a poorly adjusted derailleur, shifting may result in a rough transition with uncertain timing. Manipulation of the shifter lever is a distraction to the rider, usually for an undue period of time, and may result in an unsafe operating condition. The shifting of a three speed or five speed bicycle through successively higher or lower speed ranges calls only for a simple easily understood progressive position changes of the selector lever. However, for a bicycle which has two derailleurs, such as a ten speed, the typical pattern for the two selector levers is not a simple progressive positioning pattern to obtain successively higher or lower speed ranges; instead, it is a complex shift pattern which the rider must memorize in order to select a desired speed range.
In the field of bicycling, there has been a long-standing need for replacing or supplementing the manual shifter with an automatic shifter to relieve the rider from the need for manual shifting. While the required technology, in terms of mechanics and electronics, has long been available for an automatic shifter and while there have been many inventions and proposals, none has met with any significant success.
The prior art systems are unduly complex and too costly to manufacture. They require replacement or modification of conventional component parts and are not adapted for installation on a conventional bicycle. In the prior art systems, the criteria for shifting leaves much to be desired in meeting the needs of an individual rider in producing a timely upshift or downshift with quick and smooth shifting. Further, the prior art is lacking in respect to providing highly efficient bicycle operation while also providing minimized physical and mental demands on the rider.
In the prior art, it has been known for several years to provide an automatic shifter for bicycles wherein a derailleur is actuated by an electric motor under the control of a microcomputer in response to rotational speed of a bicycle wheel and/or rotational speed of the crank gear. Such an automatic shifter is described in Matsumoto et al U.S. Pat. No. 4,490,127 granted Dec. 25, 1984. In the system of this patent, a microcomputer with a stored program derives 1) optimum running speed ranges and 2) crank gear rotation speed ranges which correspond respectively to the five transmission gears. A proper speed range is defined as that which enables the rider to ride the bicycle at maximum efficiency while being subjected to the least fatigue, taking into account the rider and various driving conditions. In the system of this patent, the transmission is shifted by a cable wound on a motor driven take-up reel and connected with the derailleur. A rotary decoder driven by the take-up reel shaft detects which transmission gear has been selected. A wheel speed sensor develops a bicycle speed signal which is supplied to the microcomputer and a crank gear rotation sensor develops a signal corresponding to the crank gear speed which is supplied to the microcomputer. The proper speed range for each transmission gear is defined between upper and lower limits which are expressed in terms of constants supplied by setting switches. Information regarding the rider and topography, along with the condition setting switches is supplied to the microcomputer. In operation under the control of the computer program, the running speed of the bicycle is compared with the proper speed range for the selected transmission gear. When the running speed is greater than the maximum speed in the proper speed range, an upshift signal causes the motor actuator to select the next higher speed range. Conversely when the actual speed is lower than the minimum speed of the proper speed range, a downshift signal is produced to select the next lower transmission gear so that the actual running speed is in the proper speed range. Also, the system of this patent includes a process for defining a crank gear rotation speed range to allow the crank gear to rotate substantially at constant speed for each transmission gear. For this, upper and lower limits for the crank gear speed are established to define a speed range for each of the five transmission gears. These limits are established by assigning constants for each of the speed ranges by the use of condition setting switches. These constants are initial values which are modified or compensated by a constant dependent upon the individual bicycle rider and the riding conditions. When the crank gear rotational speed key switch is depressed, the actual speed of rotation of the crank gear is compared with the proper speed range for the selected transmission gear. When the actual speed is higher than the maximum speed of the range, an upshift signal is generated to upshift the transmission so that crank speed is within the speed range of the selected gear. Conversely, when the actual speed is less than the minimum speed of the speed range, a downshift signal is generated to downshift the transmission so that the actual speed is within the speed range of the selected gear.
Another electronically controlled bicycle transmission is described in the Clem et al U.S. Pat. No. 4,605,240 granted Aug. 12, 1986. In the system of this patent, a microcomputer receives inputs from a bicycle speed detector, a plurality of manually controlled switches and a feedback signal from an actuator for the derailleur. The actuator is controlled by the microcomputer and positions the derailleur for gear selection in accordance with bicycle speed.
An automatic transmission for a multi-speed bicycle is also described in the Dutil et al U.S. Pat. No. 4,701,152 granted Oct. 20, 1987. The system of this patent shifts the axial position of the derailleur automatically in response to the speed of the bicycle wheel which is sensed by centrifugal fly weights on the wheel. Other prior art patents which describe automatic shifters for bicycles which effect shifting in response to bicycle speed are as follows: Gardel et al U.S. Pat. No. 3,830,521 granted Aug. 20, 1974 describes an automatic shifter which uses a fly weight governor for shifting the transmission in response to bicycle speed. The Stuhlmuller et al U.S. Pat. No. 3,919,891 granted Nov. 18, 1975 discloses a shifter which has an electrical actuator with manual switching for speed range selection and is provided with a speed responsive inhibiting arrangement to prevent shifting until a certain speed is reached. The Dantowitz U.S. Pat. No. 3,926,020 granted Dec. 16, 1975 discloses a bicycle transmission with a belt drive and variable diameter pulley with a hydraulic actuator controlled by a fluid pressure speed signal corresponding to pedal speed.
It is also known in the prior art to provide an automatic shifter for a bicycle in which the ratio is changed in response to the value of torque applied to the power train of the bicycle. The McGuire U.S. Pat. No. 3,769,848 granted Nov. 6, 1973 discloses an automatic shifter having a set of sprockets on the rear wheel which is laterally shiftable, in response to torque, to change the engagement of the chain from one sprocket to another. In this device, a cam track is provided on a shaft which supports the hub and sprockets for axial movement; a cam follower is carried by the hub so as to move the hub axially against a spring as the torque on the hub increases. The hub is moved axially in the opposite direction by the spring as the torque decreases.
In the Perry U.S. Pat. No. 3,929,025 granted Dec. 30, 1975, a mechanical linkage arrangement is provided for actuating a derailleur in response to changes of chain tension resulting from variations in pedal pressure. In the device of this patent a control arm is pivotally mounted on the frame and connected with the derailleur by a cable. An idler sprocket, which is mounted on the control arm in engagement with the chain, causes rotation of the control arm in one direction with increasing chain tension. The control arm is rotated in the opposite direction by a spring in response to decreasing chain tension. The control arm is in equilibrium for a predetermined chain tension in each of the multiple positions of the arm. When the actual chain tension is different from the predetermined tension, the control arm rotates in a direction to eliminate the difference and shifts the derailleur to a different speed range.
Also, in the prior art, it has been proposed to provide an automatic shifter for bicycles which is actuated in response to bicycle speed at a speed value which is modified by the driving torque applied to the traction wheel. Such a device is described in the Imhoff U.S. Pat. No. 4,713,042 granted Dec. 15, 1987. In the device of this patent, a shifter sleeve carries a set of sprockets and is axially slidable in the hub of the traction wheel. A set of fly weights produces a shifting force to move the shifter sleeve against a bias spring in one direction in response to increasing speed to change the chain engagement from one sprocket to another. It is moved in the other direction by the bias spring in response to decreasing speed. The shifter sleeve is drivingly engaged with the hub of the traction wheel by a cam slot in the hub and a cam follower on the sleeve. Increasing torque on the shifter sleeve produces an increasing resistance to the movement of the shifter sleeve in the axial direction and thus modifies the value of speed at which the chain is shifted from one sprocket to another.
A general object of this invention is to provide an improved automatic shifter for velocipedes, especially bicycles, and to overcome certain disadvantages of the prior art.